1. Field of the Invention
The present invention relates to a rare earth bonded magnet, and more specifically to a rare earth bonded magnet suitable for use in a hard disk drive.
2. Description of the Related Art
A rare earth bonded magnet is produced such that a mixture of rare earth magnetic powder and thermosetting resin such as epoxy resin (the mixture will hereinafter be referred to as “compound”) is filled in a mold and compressed into a green compact, which is then heat-cured. Such a rare earth bonded magnet is required to fulfill industrial productivity, dimensional accuracy, and corrosion resistance as well as magnetic property, and when used in a spindle motor for a hard disk drive it is requested that outgas and other foreign particles be restrained and reduced which contaminate a recording medium. The magnetic property is influenced by the characteristic of magnetic powder, the proportion of magnetic powder and binder, and the compact density, and the industrial productivity is influenced by mechanical strength at the time of attachment to a motor. As for the corrosion resistance, good oxidation resistance and moisture resistance inhibit the magnetic property from degrading and the magnetic powder from shattering. Also, good corrosion resistance allows the thickness of coating to be reduced thereby enhancing the dimensional accuracy. And, in the aspect of moldability, the powder density, mold-fillability (flowability, uniform fillability), and mold-releasability are important factors. Among these factors, the powder density is crucial to producing a lengthy compact, and a compound with a higher powder density allows the filling depth into a mold to be reduced thereby ensuring the precision of the mold with less difficulty. Also, high fillability enables enhancement of the dimensional accuracy of a green compact, and application of a resultant magnet, for example, in a motor will reduce vibration.
It is known that a compound comprising only magnetic powder and thermosetting resin cannot achieve satisfactory moldability, and therefore metallic soap, for example, calcium stearate and zinc stearate, is conventionally applied as lubricant for enhancing moldability (refer to, for example, Japanese Patent Application Laid-Open No. H11-045816). When lubricant such as the aforementioned calcium stearate and zinc stearate is added to the compound, the mold-fillability (flowability, uniform fillability) and the mold-releasability are improved as well as the powder density is increased, and also, since the friction between the particles of the compound is reduced, the compressibility is improved, and there by the green compact density is increased. Fluorinated resin powder is proposed as an alternative lubricant (refer to, for example, Japanese Patent Application Laid-Open No. 2000-036403). Fluorinated resin has a high melting point (over 320 degrees C.) and does not melt when a green compact is subjected to heat-curing treatment thus having a deterrent effect on degradation in mechanical strength of a resultant product.
When the metallic soap such as the calcium stearate or the zinc stearate is used as lubricant as disclosed in the aforementioned Japanese Patent Application Laid-Open No. H11-045816, the lubricant melts at the time of heat-curing treatment because it has a melting point lower than the temperature (100 to 200 degrees C.) of the heat-curing treatment of thermosetting resin used in compression molding, which results in a significantly degraded mechanical strength of the resultant product compared when such a lubricant is not used. If a reduced amount of lubricant is added to prevent this problem, then satisfactory moldability, which is the very purpose in adding lubricant, cannot be achieved.
In a hard disk drive, a magnetic read/write head floats over a recording medium, and outgas and contaminations on the recording medium cause problems. Consequently, the specifications of a rare earth bonded magnet for use in a hard disk drive are stringent with respect to outgas and ion residue. It is difficult for a rare earth bonded magnet containing the fluorinated resin as lubricant as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2000-036403 to meet the stringent specifications with respect to outgas and ion residue for use in a hard disk drive, and its application scope is restricted.